When making a continuous material web, such as paper on a paper machine, for example, the material finished by the machine and rolled onto a reel is often processed with post-processing machines. Examples of such post-processing machines related to the post-processing of paper include various off-line calenders, winders, and rewinders. Common features of these post-processing machines are that the material is unwound with an unwinder, processed in a desired manner, and rewound with a rewinder.
Increasing the speeds of paper machines and corresponding machines producing a continuous material web also produces pressure to increase the speeds of the post-processing devices and to minimize shutdown times. One problematic point in post-processing devices is associated with the operation of unwinders during a material web break. The situation is especially pronounced in winders that make customized reels, that is, smaller reels ordered by the customer, from machine reels. On a machine reel, the paper or corresponding material has the width of the production machine, and the width is cut into customer-specified narrower and smaller reels by using a winder.
Several customized reels are typically obtained from one machine reel, in other words, the unwinder of the winder needs to be stopped several times during one machine reel to remove the finished customized reels and to start new ones. Even though the winder speeds are great, the releases of customized reels from them slow the average speed. If a malfunction stopping the operation of the winder occurs, the winder may become a part that contributes to slowing down of the production of the paper mill.
One issue in post-processing devices is associated with stopping the machine reel of an unwinder during a web break. Material is unwound from the unwinder at great speed, and in a normal situation, the material is wound with a rewinder after processing. If the material web breaks between the unwinder and rewinder, the material will unwind on the floor of the mill hall. A significant amount of material unwinding at great speed accumulates quickly on the floor, and before the restart of production, this material must be collected from the floor. When a web break is detected, the unwinder is stopped as quickly as possible to minimize the break time and material loss.
Stopping unwinders during web break situations is achieved in accordance with the prior art by using a mechanical brake at its full capacity. In addition, a motor is used for decelerating when the deceleration torque exceeds the capacity of the mechanical brake. The deceleration torque provided by the motor is limited in such a manner that the total deceleration torque does not exceed the limits of the system mechanics. A mechanical brake is obligatory equipment defined by standards, with which a machine reel in an unwinder is made to remain unrotational, and the machine reel may be stopped in a specific time.
An issue with the mechanical brake is stopping the unwinder at high rotation speeds, in particular. The mechanical brake decelerates at a fixed deceleration torque, so at high rotation speeds the power used in deceleration easily damages the mechanical brake and makes it unusable. The mechanical brake also cannot be adjusted in a simple manner so that deceleration power could be increased or decreased during deceleration. Thus, a web break may cause a long shutdown, when the brakes of the unwinder become unusable during one deceleration.
In deceleration, mechanical brakes transform the energy bound to the machine reel into heat that heats the brakes. This heating of the brakes may also cause a fire hazard, as the material releases uncontrollably from the unwinder, whereby dry paper or the like may come into contact with the heated brakes.